How do I set Maximum emission strength?
As the name implies the Maximum operations picks the maximum of the two input values, meaning it chooses the greatest of the two.

In your case by calculating the maximum between $10$ and $76.3$ you obtain a resulting intensity value of the highest input, hence  $76.3$.

From what what you describe you seem to want the opposite operation. If you set it to Minimum it will pick the smallest value of the two, effectively limiting emission to $10$, or whichever value is lowest.